1. Field of the Invention
The invention relates to a method for shaping hollow boards that are comprised of thermoplastic material and have at least one cavity within the board interior being open toward the edge of the board, wherein the cavity in the interior of the board is sealed at the board edge and is thus closed off to the exterior in a substantially gas-tight way and wherein the hollow board to be shaped is softened by heating and is shaped in the softened state by external loading with a shaping pressure while the cavity is closed off in a substantially gas-tight way.
2. Description of the Related Art
Known thermoplastic hollow boards are comprised of two parallel cover layers as well as spacers arranged therebetween, for example, in the form of separating stays or cup-shaped projections of an intermediate layer provided between the cover layers. In this connection, in the interior of the hollow boards, i.e., between their cover layers, cavities are provided which are open toward the board edge. For the shaping process, such thermoplastic hollow boards open toward the edge are softened by supplying heat, as is known in the art, and are loaded with an external shaping pressure in the softened state, for example, by means of deepdrawing devices or presses. Because of the destabilization of the hollow board in itself resulting from softening, in particular, of the spacers arranged between its cover layers, the shaping of the hollow boards according to the methods of the prior art or by means of the known devices often entails an undesirable change of the board structure, primarily an undesirable irreversible compression of the hollow boards perpendicularly to the cover layers.
A method of the aforementioned kind is disclosed in U.S. Pat. No. 4,172,749. Here, the hollow boards are shaped with cavities being open toward the edge. The cavities are filled with a gaseous medium, for example, air. Before the start of the actual shaping process, the gas-filled cavities are closed in a gas-tight way at the edge of the hollow board. The hollow board is finally shaped with the cavities being closed in this way. The gaseous medium present within the interior of the cavities thus builds up a pressure which counteracts the external shaping pressure and is supposed to prevent undesirable compression of the hollow board to be shaped.